This invention relates to printing apparatus and in particular to thermal printing apparatus wherein ink is transferred from a layer of ink of an ink ribbon by the action of heat on the ink layer.
In some forms of printing apparatus it is necessary to obtain close engagement of the print receiving medium with an ink donating element or ribbon to achieve reliable printing.
One example of such printing apparatus operating in this manner is printing apparatus in which the surface of a print receiving medium is brought into engagement with a print die which may be carried on a rotatable print drum, the print die having been inked prior to engagement thereof by the print receiving medium.
Another example is thermal printing apparatus. Thermal printers are known in which a print receiving sheet is located adjacent a thermal print head with an ink transfer ribbon interposed between the sheet and the print head. Pressure is applied to the sheet to urge the sheet into ink transfer engagement with an ink layer carried by a substrate of the ink ribbon and to urge the substrate of the ink ribbon into heat receiving engagement with thermal printing elements of the print head. The thermal printing elements are heatable selectively by passing electric current therethrough and the heating of the elements results in heating of areas of the ink layer adjacent to heated elements. Heating of the ink layer results in the ink layer adhering more strongly to the surface of the print receiving sheet than to the substrate of the ribbon. Consequently when the used ribbon is peeled from the print receiving sheet, areas of ink layer corresponding to heated elements of the print head remain adhered to the print receiving sheet while the remainder of the ink layer remains adhered to the substrate of the ribbon. The thermal elements are disposed in a column and hence after selective heating of the elements, dots of ink are adhered to the sheet in selected locations in a column. By causing relative movement between the sheet and the print head in a direction transverse to the row of elements and by repeatedly selectively energising the elements, dots of ink are adhered to the sheet column-by-column and a required printed pattern is built up column-by-column. The pattern may be alphanumeric or other characters or may be a pictorial pattern.
It is known to use either rotatable drum print heads or thermal ink transfer printers in postage meters for printing postal indicia on mail items. The indicia generally comprises an invariable pattern together with variable postage data. The variable postage data includes a value of postage charge for the mail item and the date on which the mail item is entered into the postal system. The form of the postal indicia is authorised for use by the postal authority.
In printers for printing on a sheet of paper there is little difficulty in maintaining the sheet in the required engagement with the ink donating element. In thermal printers the sheet can be readily maintained in engagement with the ribbon and the ribbon with the elements of the print head to ensure reliable printing on the sheet. However mail items may not be of uniform thickness and in postage meters it is difficult to ensure that the entire extent of the surface onto which printing is to be effected is subjected to pressure to maintain the surface in engagement with the print die or with the ink layer of the ribbon. Mail items generally consist of an envelope containing inserts. Often the inserts are of generally uniform thickness over the extent of the envelope and hence the overall thickness of the mail item, i.e. the thickness of the inserts together with the thickness of the walls of the envelope, is substantially uniform. Reliable printing can be obtained with such uniform thickness mail items. However some mail items contain inserts of non-uniform thickness. Other mail items contain inserts of less extent than the envelope. Consequently the thickness of some parts of the mail item is determined by the thickness of the inserts and the walls of the envelope while the thickness of other parts of the mail item, to which the inserts do not extend, is determined solely by the walls of the envelope.
The pressure for maintaining the sheet, ribbon and print head in the required engagement with one another is applied between the print head and roller supporting the print receiving sheet. In postage meters where the mail item is fed past a stationary print head, the roller is resiliently mounted and is urged toward the print head. The resilient mounting of the roller enables mail items of different thicknesses to be accommodated. It has been proposed that the roller be deformable at discrete intervals. However if the roller is sufficiently hard to ensure feeding of the mail item at a substantially uniform speed when the roller is rotated at a uniform speed, the roller is insufficiently deformable to apply the required pressure to all parts of a mail item in which there is significant variation of thickness.